Rheumatoid arthritis (“RA”) is a chronic, inflammatory, systemic disease that produces its most prominent manifestations in affected joints, particularly those of the hands and feet. The onset of rheumatoid arthritis can occur slowly, ranging from a few weeks to a few months, or the condition can surface rapidly in an acute manner.
RA has a worldwide distribution and involves all ethnic groups. Although the disease can occur at any age, the prevalence increases with age and the peak incidence is between the fourth and sixth decade. The prevalence estimates for the North American population vary from 0.3% to 1.5%. Today, over 2,500,000 individuals are diagnosed with rheumatoid arthritis in the United States alone, with some statistics indicating from 6.5 to 8 million potentially afflicted with the disease. Women are affected 2-3 times more often than men.
The early symptoms of rheumatoid arthritis are mostly joint specific such as painful joints with joint swelling or tenderness, but may also include rather non-specific manifestations like stiffness, fever, subcutaneous nodules, and fatigue. Very characteristic is the symmetric involvement of joints. The joints of the hands, feet, knees and wrists are most commonly affected, with eventual involvement of the hips, elbows and shoulders. As the disease progresses, any type of motion becomes very painful and difficult leading eventually to a loss of function of the involved joints. The more severe cases of rheumatoid arthritis can lead to intense pain and joint destruction. Some 300,000 bone and joint replacement surgical procedures are performed annually in an effort to alleviate the pain and mobility loss resultant from arthritis related joint destruction.
The most widely used system to classify RA is the American College of Rheumatology 1987 revised criteria for the classification of RA (Arnett, F. C., et al., Arthritis Rheum. 31 (1988) 315-324). According to these criteria (known as ARA-criteria), a patient is said to have RA if the patient satisfies at least four of the following seven criteria, wherein criteria 1-4 must be present for at least six weeks: 1) morning stiffness for at least one hour, 2) arthritis of three or more joint areas, 3) arthritis of hand joints, 4) symmetrical arthritis, 5) rheumatoid nodules, 6) serum rheumatoid factor (“RF”), and 7) radiographic changes. These criteria have a sensitivity and specificity of approximately 90%.
The histological changes in RA are not disease-specific but largely depend on the organ involved. The primary inflammatory joint lesion involves the synovium. The earliest changes are injury to the synovial microvasculature with occlusion of the lumen, swelling of endothelial cells, and gaps between endothelial cells, as documented by electron microscopy. This stage is usually associated with mild proliferation of the superficial lining cell layer. Two cell types constitute the synovial lining: bone marrow derived type A synoviocyte, which has macrophage features, and mesenchymal type B synoviocyte. Both cell types contribute to synovial hyperplasia, suggesting a paracrine interaction between these two cell types. This stage of inflammation is associated with congestion, oedema, and fibrin exudation. Cellular infiltration occurs in early disease and initially consists mainly of T lymphocytes. As a consequence of inflammation, the synovium becomes hypertrophic from the proliferation of blood vessels and synovial fibroblasts and from multiplication and enlargement of the synovial lining layers.
Granulation tissue extends to the cartilage and is known as pannus. The tissue actively invades and destroys the periarticular bone and cartilage at the margin between synovium and bone, known as erosive RA.
The articular manifestations of RA can be placed in two categories: reversible signs and symptoms related to inflammatory synovitis and irreversible structural damage caused by synovitis. This concept is useful not only for staging disease and determining prognosis but also for selecting medical or surgical treatment. Structural damage in the typical patient usually begins sometime between the first and second year of the disease (van der Heijde, D. M., et al., Br. J. Rheumatol. 34, Suppl. 2 (1995) 74-78). Although synovitis tends to follow a fluctuating pattern, structural damage progresses as a linear function of the amount of prior synovitis.
The aetiology of the early events in RA remains elusive. An autoimmune component is widely accepted today but other factors are still disputed. The possibility of a bacterial or viral infection has been vigorously pursued. All efforts to associate an infectious agent with RA by isolation, electron microscopy, or molecular biology have failed. It is possible that there is no single primary cause of RA and that different mechanisms may lead to the initial tissue injury and precipitate synovial inflammation.
Clinical signs of synovitis may be subtle and are often subjective. Warm, swollen, obviously inflamed joints are usually seen only in the most active phases of inflammatory synovitis. Cartilage loss and erosion of periarticular bone are the characteristic features of structural damage. The clinical features related to structural damage are marked by progressive deterioration functionally and anatomically. Structural damage to the joint is irreversible and additive.
Data from longitudinal clinical and epidemiologic studies provide guidelines for treatment. These studies emphasize 1) the need for early diagnosis, 2) identification of prognostic factors, and 3) early aggressive treatment. Earlier diagnosis and treatment, preferably within the first several months after onset of symptoms, may help prevent irreversible joint damage.
The effective treatment of rheumatoid arthritis generally comprises a combination of medication, exercise, rest and proper joint protection therapy. The therapy for a particular patient depends on the severity of the disease and the joints that are involved. Non-steroidal anti-inflammatory drugs, corticosteroids, gold salts, methotrexate and systemic immunosuppressants are widely used to reduce inflammation and joint destruction. The use of steroids and immunosuppressants, however, has significant risks and side effects both in terms of toxicity and vulnerability to potentially lethal conditions. More recently therapeutics based on “biologicals” have been introduced into RA-therapy. Such therapeutics, e.g., are soluble receptors or antibodies directed against TNF-α that significantly reduce inflammation. Though very promising, biologicals are still in limited use due to high costs.
The ideal scenario for establishing a diagnosis or assessing the risk of disease progression would be a situation wherein a single event or process would cause the respective disease as, e.g., in infectious diseases. In all other cases correct diagnosis can be very difficult, especially when the etiology of the disease is not fully understood as is the case for RA. Therefore in RA, generally various clinical symptoms and biological markers are considered together for diagnosis of RA or for assessing the risk of disease progression.
The first biochemical marker and the only one generally accepted (see the above ARA-criteria) for aiding in the diagnosis of RA is the rheumatoid factor (RF) as detected in serum. Recently a novel marker called anti-CCP has been introduced. It has been confirmed in many independent studies that autoantibodies to cyclic citrullinated peptides (anti-CCPs) represent a highly sensitive and specific marker for diagnosis of RA.
Anti-CCPs have been intensively studied during the past years by several groups of researchers (cf., e.g., WO 98/08946; WO 98/22503; WO 99/28344; WO 99/35167; WO 01/46222; and WO 03/050542). Recently Schellekens and co-workers (Schellekens, G. A., Arthritis Rheum. 43 (2000) 155-163) reported that an ELISA-test based on specific cyclic citrullinated peptides (CCP) showed superior performance characteristics with regard to diagnostic accuracy for RA as compared to the same assay using linear peptides.
Auto-antibodies against CCP, i.e., antibodies which most likely are reactive with citrullinated polypeptides circulating in a patient serum and which bind to CCP in an in vitro assay are termed “anti-CCP”. The patent application of van Venroji et al. (WO 98/22503) describes certain citrullinated peptides and shows that cyclization leads to an improved reactivity of autoantibodies to the these peptides. By using improved CCPs as an antigen for detection of anti-CCP antibodies the sensitivity is increased to 63% as compared to 36% to the corresponding linear peptides. Since autoantibodies in patient sera have slightly different reactivity to different cyclic peptides a combination of peptides was suggested in WO 98/22503 to further improve the assay.
Many research groups have recently shown and confirmed that anti-CCP is an even more sensitive and specific marker for establishing the diagnosis of RA as compared to RF. Anti-CCP autoantibodies are highly specific for RA (ca. 97% specificity), with a sensitivity comparable to that of RF (65-80%) (Lee, D. M. and Schur, P. H., Ann. Rheum. Dis. 62 (2003) 870-874; Pruijn, G. J. M., et al., Curr. Rheumatol. Rev. 1 (2005) 1-7; Vallbracht, T., et al., Ann. Rheum. Dis. 63 (2004) 1079-1084). Furthermore it is of additional diagnostic value that anti-CCP can be detected in a significant percentage of seronegative RA patients (van Paassen, P., et al., Best Pract. Res. Clin. Rheumatol. 17 (2003) 475-494; Vallbracht, I., et al., Ann. Rheum. Dis. 63 (2004) 1079-1084; Schellekens, G. A., et al., Arthritis and Rheumatism 43 (2000) 155-163). This means that anti-CCP autoantibodies are present in a significant fraction of patients (sero-) negative for RF.
As discussed above, establishing a diagnosis of RA and deciding for the optimal treatment option is not an easy task. The course of disease in individual RA patients varies significantly. No unique and generally accepted set of indicators for poor outcome in RA exists to date. Indicators associated with a bad prognosis include e.g. cumulative joint inflammation, high ESR or CRP levels, RF positivity, early radiological erosions, poorer scores for function and adverse socioeconomic circumstances.
To make things even more complicated, assessing a prognosis in RA also suffers from a lack of a clear and generally accepted definition of disease progression.
Several scores—on the basis of clinical symptoms, radiographic changes or physical function—have been developed in order to assess treatment response in RA. However, most of these scores are used in clinical trials settings only, but rarely or not at all in rheumatology practices. Examples are the different response criteria devised by the American College of Rheumatology (ACR) and the European League against Rheumatism (EULAR) (Felson, D. T., et al., Arthritis and Rheumatism 38 (1995) 727-735; van Gestel, A. M., et al., Arthritis and Rheumatism 39 (1996) 34-40). Both—the ACR improvement criteria and the EULAR response criteria—are widely used in clinical trial settings, but not in clinical practice. The same is true for the scoring systems for assessment of radiographic changes according to Sharp or Larsen and several modifications thereof are available to date. Although X-rays are taken for monitoring of radiographic disease progression at regular intervals, they are only compared to previous X-rays but not scored.
Furthermore, in Europe the DAS (disease activity score) and simplifications thereof (DAS28, SDAI, CDAI) are widely used for disease monitoring under therapy. The DAS includes tender and swollen joint counts, ESR or CRP and a global assessment of disease activity (using a VAS—visual analog scale). To a minor extent also assessments of physical function do play a role in monitoring of disease states. These are based on different patient questionnaires—the most widely used in RA being the HAQ (Health Assessment Questionnaire) (Bruce, B. and Fries, J. F., Health Qual Life Outcomes 1 (2003) 20) and the SF-36 (Short Form 36) (Talamo, J., et al., Brit. J. Rheumatol. 36 (1997) 463-469).
However, the above mentioned tools are far from optimal. They are time-consuming and influenced by subjective assessments, e.g. in case of the HAQ or tender/swollen joint counts.
Recently, attempts have been made to further assess various aspects of RA by including more biochemical markers into such assessment or to even base such assessment on biochemical markers.
Coste, J., et al. (The Journal of Rheumatology 24 (1997) 28-34) have investigated twenty clinical and laboratory parameters for their ability to predict articular destruction in RA. Statistical significant correlations to disease progression were found for iron, CRP, ESR, and α1-acid glycoprotein. However, correlations were not very strong and only existing for the first 6 months of follow-up.
Aman, S., et al., Rheumatology 39 (2000) 1009-1013 investigated whether disease progression in RA could be predicted by the markers ICTP, RF and CRP. They found odds ratios from 2.6 to 3.9 for the individual markers and the best marker ratio had an odds ratio of 9.1. This odds ratio translated to a specificity of 71% at a sensitivity of 77%. However, a specificity of 71% is rather low, because in clinical routine a specificity of at least 80%, or preferably even of at least 90% is required.
Visser, H., et al., Arthritis and Rheumatism 46 (2002) 357-365, have proposed “A prediction model for persistent (erosive) arthritis”. Their model consists of The developed prediction model consisted of 7 variables: symptom duration at first visit, morning stiffness for ≧1 hour, arthritis in ≧3 joints, bilateral compression pain in the metatarsophalangeal joints, rheumatoid factor positivity, anti-cyclic citrullinated peptide antibody positivity, and the presence of erosions (hands/feet). As can be seen two biochemical markers, RF and anti-CCP, formed part of their algorithm.
Recently, Meyer, O., et al. (Arthritis Research and Therapy 8/2 (2006) R40), have proposed to use serial determinations of anti-CCP autoantibodies to predict the radiological outcomes after five years of follow-up. They demonstrated that the determination of anti-CCP at baseline is not a sufficient predictor of disease progression. However an aid in the prediction of progression at baseline is exactly what is needed by the practitioner.
Whereas both RF and anti-CCP are important tools in establishing the diagnosis of RA, they appear to be not a strong aid in predicting the future course of disease. Many markers or sets of markers have been proposed, however the odds ratios achieved thus far have not been sufficient or have been based on a too large variety of biochemical and clinical parameters to meet clinical routine requirements.
Hence there is a tremendous need for a method, especially based on biochemical parameters, aiding in assessing whether an RA patient is at risk of disease progression or not.
It now has been surprisingly found that the two markers CRP and interleukin-6 supplement each other and thus lead to an improvement in the assessment of a patient's risk to undergo a more severe course of RA. The present invention is expected to at least partially overcome the problems existing in the field of assessing whether an RA patient is at risk of disease progression by providing methods and reagents for assessing whether an RA patient is at risk of disease progression in vitro.